1. Field of the Invention
The present invention relates to an incandescent lamp of which the lifetime is extended by supplying the tungsten filament of the lamp through two rectifying diodes and a resistor. More specifically, the present invention is concerned with, in particular but not exclusively, incandescent lamps of the type used in traffic signal lights and in high structure obstruction marking.
2. Brief Description of the Prior Art
Conventionally, A-line or PS-line gas filled incandescent lamps are used in traffic signal lights and in high structure obstruction marking. Such lamps are constructed to operate from an AC (alternating current) voltage having a nominal amplitude of 120 volts and a nominal frequency of 60 Hz, and have standard power ratings of 69, 90, 100, 105, 116, 620 or 700 watts. These prior art incandescent lamps, having an average lifetime of 5000 hours, may be termed short-lifetime lamps as compared with known incandescent lamps having extended lifetimes up to several years. Failure of these conventional incandescent lamps is mainly caused by evaporation of the tungsten of the filament, the rate of evaporation of the tungsten increasing with the temperature of operation of the filament.
To extend the lifetime of an incandescent lamp, U.S. Pat. No. 4,897,578 granted to Christopher F. Goad, Sr. on Jan. 30, 1990 uses a rectifying diode connected in series with the tungsten filament of the lamp. Incorporation of a rectifying diode in the structure of an incandescent lamp is further described in the following three United States patents:
______________________________________ 3,148,305 Pearson 1964 3,869,631 Anderson et al. 1975 4,480,212 Monahan 1984 ______________________________________
The use of a rectifying diode installed in a socket for extending the lifetime of incandescent lamps is also disclosed in the following two United States patents:
______________________________________ 3,450,893 Munson 1969 3,963,956 Beining 1976 ______________________________________
With a rectifying diode, only the positive or negative half-cycles of the AC voltage are applied across the filament. Consequently the effective voltage supplying the tungsten filament is substantially lowered, thus reducing the temperature of operation of the filament. For example, when no rectifying diode is connected in series with its tungsten filament, a standard above-mentioned incandescent lamp of 100 watts has an operating temperature of approximately 2590.degree. C. and produces a light intensity of about 1070 lumens. With a rectifying diode connected in series with the tungsten filament, the operating temperature of the same lamp reduces to 2190.degree. C., and the light intensity produced is approximately 375 lumens. Connection of a diode in series with the tungsten filament of an incandescent lamp reduces the power consumption of the lamp from 100 watts to 58.4 watts and increase its average operating lifetime from about 3000 hours to approximately 9.7 years.
However, connecting a rectifying diode in series with the tungsten filament of an incandescent lamp presents the drawback that the tungsten filament has a lower efficiency in terms of lumens per watt.
Another drawback of connecting a single rectifying diode in series with the filament of an incandescent lamp as taught in the above mentioned U.S. Pat. No. 4,897,578 (Christopher F. Goad, Sr.) issued on Jan. 30, 1990, is that the incandescent lamp is no longer compatible with the traffic signal light monitoring control systems and with the high structure obstruction marking light alarm systems presently in operation. Indeed, a traffic signal light monitoring control system indicates proper operation of a traffic signal lamp upon detection of both positive and negative half-cycles of an AC current flowing through the tungsten filament of this lamp. In the same manner, a high structure obstruction marking light alarm system must detect both positive and negative half-cycles of an AC current flowing through the tungsten filament, otherwise an alarm indicating a faulty marking lamp is produced. Accordingly, to prevent the traffic signal light monitoring control systems and the high structure obstruction marking light alarm systems presently in operation to generate a false indication of a faulty lamp, they have to detect both positive and negative half-cycles of an AC current flowing through the tungsten filament. The rectifying diode of the incandescent lamp of Goad prevents one of the positive of negative half-cycles from being supplied to the traffic signal light monitoring control systems and the high structure obstruction marking light alarm systems.